Symptoms of seasonal affective disorder (SAD) in humans resemble seasonal changes that occur in animals. Moreover, in both humans and animals, the occurrence of these changes is regulated by light. In animals the circadian pacemaker regulates seasonal behavior by transmitting a signal of daylength to other sites in the organism. This signal is expressed, reciprocally, in the duration of nocturnal melatonin secretion, which is longer in winter and shorter in summer. Sites distal to the pineal that regulate seasonal behavior read and respond to this melatonin signal of change of season. In a longitudinal study, we showed that a homologous melatonin signal of change of season is present in patients with SAD but not in healthy volunteers. In light of the animal models, this finding raises the possibility that seasonal changes in duration of melatonin secretion govern the course of illness in patients with SAD. If so, then an intervention that shortens the duration of melatonin secretion in winter and thereby mimics the effect of summer days should improve symptoms of winter depression. To test this hypothesis, we propose to administer propranolol, a beta-adrenergic receptor blocking medication that is frequently prescribed for the treatment of hypertension, to patients with SAD in the winter. We will administer propranolol at a time of night when it would suppress and shorten the duration of melatonin secretion and then ascertain whether this intervention improves depression. A unique feature of this parallel-design, controlled clinical trial is that propranolol, when administered at a time of day when melatonin is not secreted, can serve as its own active placebo.